Spaceship 1 and Spaceship 2 have equal masses of 300 kg. They collide. Spaceship 1's final speed is 2 m/s, and Spaceship 2's final speed 1 m/s in the same direction. What is their combined momentum?
A. 600 kg-m/s
B. 900 kg-m/s
C. 1,200 kg-m/s
D. 3 m/s